The production of industrial-scale volumes of hydrogen may be accomplished by application of the steam-methane reforming process, which entails the catalytic reforming of natural gas with steam at elevated temperatures (800-900° C.). This process yields a crude synthesis gas, which is a mixture of hydrogen, carbon monoxide, and carbon dioxide, and the crude synthesis gas is further reacted in a catalytic water-gas shift conversion step to convert carbon monoxide and water to additional hydrogen and carbon dioxide. The shifted synthesis gas is purified to yield a final hydrogen product containing greater than 99 vol % hydrogen.
The natural gas reforming reaction is highly endothermic, requiring about 45 kcal/mole of methane reformed, and the productivity of the steam-methane reforming process is limited by the rate of heat transfer from the external heat source to the catalyst. The catalyst typically is contained in long metal alloy tubes, and the alloy is selected to withstand the elevated temperatures and pressures required by the process. A significant part of the capital cost of the steam-methane reforming process equipment is related to the need for significant heat transfer at the high operating temperatures and pressures.
An alternative process for the production of hydrogen is the partial oxidation of methane to form synthesis gas, which is subsequently shifted if necessary and purified by pressure swing adsorption (PSA). Partial oxidation is known to be highly exothermic. Another alternative process to generate synthesis gas for hydrogen production is autothermal reforming, which is essentially a thermally balanced combination of the steam-methane reforming process and partial oxidation. One considerable drawback associated with these alternative processes is that partial oxidation requires a supply of high purity oxygen gas to the reaction system. Therefore, the use of these processes requires the additional step of separating air to produce the oxygen gas, and the air separation process increases the capital and operating costs of hydrogen production.
Numerous cyclic methods for the production of hydrogen gas are known in the art. One method entails the reaction of metal oxides with steam and methane. U.S. Pat. No. 6,761,838 describes the production of hydrogen and carbon monoxide by the partial oxidation and/or steam reforming of hydrocarbons in an autothermal process. The publication further discloses the use of an oxygen ion conducting, particulate ceramic in a cyclic process which involves the reaction of oxygen in the air feed with the ceramic in one (regeneration) step, and the reaction of hydrocarbon feed and, optionally, steam, with the oxygen-enriched ceramic produced in the first step, to produce hydrogen and carbon monoxide (hydrogen production step). Preferred ceramic materials are stated to include perovskite substances.
Investigations of the catalytic steam-methane reforming reaction have been carried out in systems which contain carbon dioxide acceptors to yield a higher-purity hydrogen rich product. For example, the use of calcium oxide, a carbon dioxide acceptor which is converted to calcium carbonate by chemisorption of the carbon dioxide, is disclosed in “The Process of Catalytic Steam-Reforming of Hydrocarbons in the Presence of Carbon Dioxide Acceptor,” A. R. Brun-Tsekhovoi et al., Hydrogen Energy Progress VII, Proceedings of the 7th World Hydrogen Energy Conference, Moscow, Vol. 2, pp. 885-900 (1988). The use of calcium oxide as a carbon dioxide acceptor in the steam-methane reforming reaction is also disclosed in “Hydrogen from Methane in a Single-Step Process,” B. Balasubramanian et al., Chem. Eng. Sci. 54 (1999), 3543-3552. Hydrotalcite-based carbon dioxide adsorbents are disclosed in “Adsorption-enhanced Steam-Methane Reforming,” Y. Ding et al., Chem. Eng. Sci. 55 (2000), 3929-3940.
U.S. Pat. No. 5,827,496 discloses a process for carrying out an endothermic reaction, such as the reforming petroleum hydrocarbons, within a packed bed reactor using an unmixed combustion catalytic material and a heat receiver. The catalytic materials are referred to as “mass-transfer catalysts,” and include metal/metal oxide combinations such as nickel/nickel oxide, silver/silver oxide, copper/copper oxide, cobalt/cobalt oxide, tungsten/tungsten oxide, manganese/manganese oxide, molybdenum/molybdenum oxide, strontium sulfide/strontium sulfate, barium sulfide/barium sulfate, and mixtures thereof. The heat receiver may also include a CO2 sorbent material, which is essentially limited to calcium oxide or a source thereof. This patent, in the context of its disclosed general process for heat transfer by “unmixed combustion,” describes a process for reforming petroleum hydrocarbons with steam. The process includes thermal regeneration and CO2 sorbent regeneration.
U.S. Pat. No. 6,007,699 also discloses an “unmixed combustion” method that utilizes a combination of physical mixtures of metal oxides, a heat receiver and a catalyst comprising one or more metal/metal oxide combinations. Examples of the heat receiver include CaCO3, boiling water, a reforming reaction in a combustion system, a catalyst system requiring regeneration, and an adsorbent or absorbent material during regeneration. Calcium oxide is used to remove carbon dioxide and drive the equilibrium reaction towards the production of hydrogen. In an embodiment, heat is supplied to a packed bed of a sorbent to thermally regenerate the sorbent.
U.S. Pat. No. 6,682,838 discloses a method for converting hydrocarbon fuel to hydrogen-rich gas by reacting the hydrocarbon feed with steam in the presence of a reforming catalyst and a carbon dioxide fixing material, removing carbon monoxide from the hydrogen gas product by methanation or selective oxidation, and regenerating the carbon dioxide fixing material by heating it to at least 600° C. Suitable disclosed carbon dioxide fixing materials include calcium oxide, calcium hydroxide, strontium oxide, strontium hydroxide, and other mineral compounds containing Group II elements.
U.S. Pat. No. 6,767,530 to Kobayashi et al. describes a method for producing hydrogen wherein steam and methane are reacted to produce synthesis gas from which hydrogen is recovered, and heat employed in the process is recovered using a defined regenerative bed system.
United States Patent Application Publication No. 2004/0191166 by Hershkowitz et al. describes a method for generating high pressure hydrogen. A synthesis gas stream is produced in a pressure swing reformer. The synthesis gas is subjected to a high temperature water gas shift process to produce a hydrogen enriched stream. Specific embodiments of the process include regenerating the reformer at a pressure lower than the synthesis gas generation.
U.S. Pat. No. 6,506,510 to Sioui et al., describes an integrated system for the co-production of heat and electricity for residences or commercial buildings based on the cracking of hydrocarbons to generate hydrogen for a fuel cell. The cracking reaction is coupled with an air or steam regeneration cycle to reactivate the cracking catalyst for further use. This regeneration can provide a valuable source of heat or furnace fuel to the system.
As described above, many cyclic processes practiced or proposed for the commercial production of hydrogen gas and/or synthesis gas include a hydrogen production step where material in the hydrogen reaction vessel is degraded and a regeneration step where the material is regenerated for a subsequent hydrogen production step.
It would be desirable to improve the thermal efficiency of hydrogen production processes having a regeneration step. The regeneration effluent gas from one reactor in a plurality of reactors in the prior art process is fed to either a heat recovery system or a gas turbine to recover its energy. While a major portion of the energy in this gas stream is recovered this way, there is still a significant portion of the energy that is lost as low level heat because the spent regeneration effluent gas has to be discharged from the plant at a greater than ambient temperature (typically greater than 250° F.). This energy loss in the form of low level heat is very similar to the energy loss in the flue gas when it leaves the stack at a conventional steam hydrocarbon reforming (SMR) hydrogen plant. It would be desirable to reduce the amount of flue gas generated from the regeneration step, thereby improving the thermal efficiency of the process.
Many of the hydrogen production processes described above also include one or more purge steps. It would also be desirable to provide any necessary purge steps with existing available gas streams without the need to generate additional steam, carbon dioxide, or importing inert gases.
It would be desirable to produce hydrogen-containing gas and be able to tolerate carbon deposition within the hydrogen reaction vessel. It would be desirable to benefit from carbon deposition within the hydrogen reaction vessel.
It would be desirable to produce hydrogen-containing gas without a pre-reformer.
It would be desirable to produce hydrogen-containing gas using sulfur-containing fuels without a sulfur removal system for removing sulfur from the fuel.
Known processes for the generation of hydrogen gas from hydrocarbons thus have associated drawbacks and limitations due to the highly endothermic nature of the hydrocarbon steam reforming reactions, feedstock purification, and the requirement of an oxygen supply for the partial oxidation of hydrocarbons used in autothermal reforming. There is a need in the field of hydrogen generation for improved process technology for the generation of hydrogen gas by the reaction of methane or other hydrocarbons with steam without certain of the limitations associated with known processes. This need is addressed by the embodiments of the present invention described below and defined by the claims that follow.